1. Field of the Invention
The present invention relates to a steering mechanism for a four wheeled vehicle. More particularly, the present invention concerns a steering mechanism for a short wheelbased four-wheeled wheelchair having a flat plate member defining four cam follower slots, two sliding members defining slots, four linkages connected to the four wheels and following the path of the cam followers and a single actuator wherein the steering mechanism enables the wheelchair to maneuver in tight spaces and minimize wheel misalignment.
2. Description of the Related Art
The ability of a wheelchair user to maneuver in tight spaces is closely related to the drive and steering configuration of the wheelchair. The most common drive configuration is the differential rear wheel drive which is illustrated in FIG. 1 and consists of fixed and driven rear wheels 10 and front caster wheels 12. Direction changes are made by individually varying the speeds of the rear wheels 10. In this configuration the point about which the wheelchair rotates lies on a line 14 which is perpendicular to and extends through the center of the rear wheels 10. The minimum turning radius is achieved when the pivot point is directly between the rear wheels 10 which is point 16. The minimum turning radius corresponds to the minimum space required to turn the wheelchair, and is determined by the maximum distance from the pivot point to any other point on the wheelchair which is usually the front corners of the wheelchair at 18 or the user's feet hanging off the front of the wheelchair.
In order to minimize the turning radius for the rear wheel differential drive configuration, several conventional wheelchair manufacturers have moved the rear wheels forward closer to the geometric center of the wheelchair resulting in moving the pivot point of the wheelchair closer to the geometric center of the wheelchair and reducing the wheelchair turning radius. A disadvantage to this approach is that it includes casters and the more weight there is on the caster wheels, the more difficult it becomes to change directions when caster wheels must flip directions. Furthermore, this approach causes the designer to take extraordinary steps to provide stability. Typically, stability is achieved by counter balancing the user's mass over and in front of the main drive wheels with the mass of the batteries behind the main drive wheels. It may be necessary to provide caster or sprung wheels in the rear of the chair to avoid tipping backwards while accelerating forward. The addition of these extra wheels may also compromise the chair's ability to climb over low obstacles if the wheels are small.
Nowhere in the related art is there disclosed or suggested a steering mechanism for a wheelchair which enables the user to maneuver the wheelchair in tight spaces, while minimizing wheel misalignment and providing the ability to climb over low obstacles. Therefore, there is a definite need for a wheelchair incorporating the steering mechanism of the present invention which provides maneuverability in tight spaces which avoids the problems associated with caster wheels such as lack of stability and poor ability to climb over obstacles.